The present invention facilitates the detection of events called packet collisions which occur on a Broadband Communication Bus (BCB) employing RF modulators and demodulators to inject and extract signals onto and off of the bus.
Broadband Communication Buses or Broadband Local Area Networks frequently establish virtual communications links over a timeshared channel. This timeshared channel is commonly called a packet service channel because of the "packetized" nature of the digital information it carries. The electronics located at the nodes where the packets are injected into and extracted from the bus are commonly called bus interface units (BIUs).
The timeshared condition of the packet channel requires a protocol to which all BIUs adhere in order to maximize channel utilization. One well known channel access protocol is called carrier sense multiple access/collision detection (CSMA/CD). The operation of the protocol is simple. A bus interface unit, when ready to transmit a packet, listens for signals on the bus and, if none are present, begins transmission of the packet. However, due to the finite propagation time delay from one end of the bus to the other, there is a chance that two or more bus interface units will begin transmitting packets simultaneously. This condition results in a packet collision. The collisions, in turn, diminish the throughput capacity of the channel.
Frequency modulation (FM) is normally used for signals being transmitted over the bus. Many of the attributes which make FM a good performing modulation on the BCB also create difficulties when attempting to reliably detect packet collisions. The most widely recognized packet service channel access protocol is defined by the Institute of Electrical and Electronic Engineers (IEEE) in IEEE Standard 802.3 CSMA/CD Access Method and Physical Layer Specifications.
In the proposed IEEE standard, the transmitting BIU compares any data existing on the bus with the data it has transmitted and if there is an error detected, it is presumed to have been due to a packet collision. To inform all bus users of the collision, the BIU of interest then transmits a signal on another channel called the collision enforcement channel. At this point, all BIUs stop transmitting and wait a random period of time before attempting retransmission.
Various disadvantages render the current collision detection technique unattractive. Primarily, the requirement for the BIU to transmit and receive on an adjacent RF channel to institute the collision enforcement channel concept is expensive. BCBs typically have many packet channels; thus, they require an equal number of collision enforcement channels. Additionally, the requirement of a more elaborate method for detecting bit errors means greater expense.
The invention disclosed herein takes advantage of the FM characteristics used for the signals being transmitted over the bus to detect packet collisions. In fact, the only requirement that the collision detection apparatus places on the signals whose collisions are to be detected is that they be FM. The other parameters such as frequency and data rate are unimportant.
The collision detection apparatus is coupled to the BCB and receives signals thereon from a particular channel. If only one signal is present from one transmitting source, the collision detector circuit produces no output signal. If, however, more than one signal is present on the channel, the collision detector generates an output signal which directs the BIU to suspend transmission. Thus, the affected BIUs stop transmitting and wait a random time before attempting retransmission.
This collision detection apparatus includes a first bandpass filter coupled to the data channel for passing only those frequencies which are intended to be present on the BCB and, thus, selects the RF packet channel of interest. This eliminates any interference from adjacent channels. A frequency shifting device is coupled to the output of the first bandpass filter which shifts the frequency of the signals a first amount if only one signal is present and at least a second amount if more than one signal is present. A second bandpass filter receives the frequency shifted output signal and has a bandpass which will prevent the signal shifted a first amount from passing through the filter, but will allow the frequency shifted a second amount to pass through the filter thereby indicating a data collision.
The frequency shifting means may be a signal splitter coupled to the output of the first bandpass filter which generates two substantially similar signals and couples them to a mixer which multiplies the two signals together to generate an output having first shifted frequency components if only one signal is present on the data channel, and second shifted frequency components if at least a second signal is also present on the data channel. As indicated previously, the second bandpass filter has a bandpass which will, in fact, pass at least some of the frequency components generated if two signals are present on the data channel, but will not pass the frequency components generated if only one signal is present on the data channel.
Thus, the invention generally relates to identification of packet collisions which occur on a Broadband Communication Bus.
The invention also relates to collision detection apparatus which is responsive to signals of the type which normally reside on an RF packet channel utilizing FM communication signals.
The invention identifies a collision by splitting the packet signals on a data channel and utilizing the split signals to generate a first frequency output if signals from only one BIU are on the data channel and a second frequency if signals from more than one BIU are on the data channel. The two frequencies are obtained by multiplying the split signal by itself utilizing a conventional multiplier such as a mixer. The multiplied signal is passed through a bandpass filter which is adjusted to pass at least some of the signal components which exist as a result of mixing two or more frequencies on the data channel, but will substantially block the signal components which exist as a result of mixing only one signal from only one BIU on the data channel. Thus, an output signal represents a data collision.